Method and apparatus for avoiding exhaust plumes



March 10, 1970 R. B. ROSENBERG ETAL 3,499,717

METHOD AND APPARATUS FOR AVOIDING EXHAUST PLUMES 3 Sheets-Sheet 1 Filed Nov h "r I am BULB TAMPERA 706%, "r

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METHOD AND APPARATUS FOR AVOIDING EXHAUST PLUMES 5 Sheets-Sheet 2 Filed Nov. 26, 1968 Jn/enforsr 0527 13 Rosenberg Scqnfprd @Z. Zl/ez'Z E I Zl/z ZZzamR. Szflaas" March 10, 1970 R. B. ROSENBERG ETAL 3,499,717

METHOD AND APPARATUS FOR AVOIDING EXHAUST PLUMES Filed Nov. 26, 1968 s Sheets-Shet 5 IE. K22 i fnv/n tors Rober .3. Rose nberg Sanford 0?. Zl/ez'Z Z'Qz'llzam R. Szagis United States Patent Int. Cl. F23j 15/00 US. Cl. 431-2 14 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for avoiding the formation of an exhaust plume when flue gases containing gaseous water are introduced to the atmosphere by raising the enthalpy or heat content of the flue gases, before introducing them to the atmosphere, to a level at which the gaseous water will not condense, regardless of the pa:ticular mixture between the flue gases and the atmosphere.

This application is a continuation-in-part of our earlier copending application Ser. No. 609,390, filed Jan. 16, 1967, and now abandoned, for Method for Avoiding Exhaust Plumes, in the names of Robert B. Rosenberg and Sanford A. Weil.

This invention relates to a novel method and apparatus for avoiding the formation of an exhaust plume which commonly forms when flue gases containing water vapor are introduced to the atmosphere at certain temperature and humidity conditions.

In many modern apartment buildings, one of the conveniences provided for tenants is control by each tenant of the heat in his own apartment. This is commonly accomplished by individual heating units in the apartments, and no central heating system is used. The individual apartment heating units used to date have generally been electric heaters. One significant disadvantage of electric heat, however, is the relatively high cost of operation.

The primary way to avoid the use of electric heaters and thereby reduce operating cost is to use a through-the- Wall vented gas space heater in each apartment which effects economy because of the significantly lower heating costs when natural gas is used. These gas heating units would be compact in size and would be installed in individualapartments. The flue gases are vented through an outside wall of each apartment in the building. Heretofore, the use of the gas space heater vented through the outside walls has been considered a significant disadvantage due to the formation of a visible exhaust plume at the vent due to certain humidity and temperature conditions of the atmosphere. This is particularly a problem in areas of the country which have many days of the year at a relatively high humidity and at a relatively low temperature, which conditions are conducive to the formation of the undesired exhaust plume. It would be highly desirable to avoid the formation of a visible exhaust plume from a gas fuel combustion device so that individual gas space heaters may be used in apartment buildings without having the apartment building bathed in exhaust plumes from the individual apartments when the ambient temperature and relative humidity are conducive to the formation of visible exhaust plumes.

The exhaust plume from a gas heater is caused upon condensation of the gaseous water in the flue gases. Natural gas or methane, the most commonly used fuel for space heaters, reacts with oxygen during combustion to form carbon dioxide and gaseous water. Other hydrocarbon fuels also form gaseous water during combustion. Flue gases also contain other gases. The oxygen used during the combustion of the fuel is supplied from air and therefore excess quantities of air, nitrogen, and small quantities of other atmospheric gases are in the flue gas, although they do not participate in the overall combustion process. Since the heat of combustion is used for space heating, a suitable heat exchanger is provided for transferring the heat of combustion from the flue products,. and this results in the decrease of the temperature of the flue products. In residential space heaters, flue products are generally exhausted to the atmosphere at a temperature of about 300-600 F. The hot flue gases mix with the cooler outside air and this results in a mixture of air and the flue products. The temperature of the mixture is between that of the outside air and the temperature at which the flue gases are exhausted. The temperature of the mixture decreases until it reaches the same temperature as the outside air.

A mixture of gases has a limited ability to hold water. This is determined by the dew point temperature of a given mixtue of gases. When the mixture temperature is lower than the dew point, the mixture can no longer hold the water and some of the gaseous water condenses to form liquid water or ice, depending upon whether the mixture temperature is above or below the freezing point of water. It is this condensation of the gaseous water which forms the undesired visible exhaust plume.

The condensation of gaseous water in the exhaust gases is greatly dependent upon the outside air temperature and the oustide air relative humidity. For example, a combustion device under one set of conditions may not exhaust flue gases which show a visible plume when the outside air is at 50 F. although it may show a visible plume at 0 F. At 0 F., the plume will be larger when the relative humidity of the outside air is than it would be at 50% relative humidity.

There are various methods by which a visible exhaust plume can be avoided. One method is to remove the gaseous water from the flue gases before there is introduction to the atmosphere. This removal may be accomplished by cooling or drying the air or destroying the gaseous water. In order to cool the air or dry the air to remove the water, equipment is required. Also, a convenient method of storing and/or disposing of the condensed water must be provided. The condensed Water would be quite corrosive because of the sulfur oxides, nitrogen oxides, and carbon dioxide dissolved in it and this creates disposal and storage problems. The *water cannot be discharged outside of the building since in cold weather, unsightly icicles would be formed. The water could not be discharged into building drainage systems because special piping would be required. Removing the water by drying is also impractical since the space heater would have to be increased in size, and an extremely large size fan would be required to evaporate the water contained in the air stream. Such additional equipment, and the cost thereof, makes this approach unsuitable. Removal of the water by destroying is quite impractical since water is very stable and extensive processing is required to dissociate water.

Another possible approach to avoid the formation of an exhaust plume is to eliminate the source of the water. This, however, would require the removal of hydrogen from the fuel, such as methane. Such a procedure requires expensive chemical processing and large quantities of energy so that eliminating the source of the water is not a practical approach.

A third possible way to avoid plume formation is to dilute the flue gases with air so that the gaseous water vapor is invisible. This is essentially the same thing that occurs in the atmosphere when a visible plume disappears.

Condensation occurs at a low dilution ratio, that is, the volume of outside air per volume of flue gas. A visible plume can be avoided only if the dilution occurs instantaneously or if it occurs in an enclosure. To provide adequate dilution, an extremely large fan, in the order of ten to fifteen times larger than a conventional furnace fan, would have to be used. If condensation is made to occur in an enclosure, the enclosure would be so large as to make such an approach impractical.

It is therefore an important object of this invention to provide a novel method and apparatus for avoiding the formation of an exhaust plume as flue gases containing water vapor are exhausted to the atmosphere wherein the disadvantages of most known possible solutions are substantially avoided.

It is also an important object of this invention toprovide a method and apparatus avoiding the formation of an exhaust plume upon introduction of flue gases containing water vapor to the atmosphere by preventing condensation of the gaseous water.

It is a further object of this invention to provide a method and apparatus for avoiding the formation of an exhaust plume from the exhaust gases of a gas space heater whereby individual gas space heaters may be used in individual apartments of large apartment buildings and yet the building will not be bathed in exhaust plumes during conditions of temperature and humidity which are conductive to plume formation.

It is yet another object of this invention to provide a method and apparatus for avoiding the formation of an exhause plume when exhaust gases containing water vapor are vented to the atmosphere by adding energy to the exhaust gases before they are introduced to the atmosphere.

It is still another object of this invention to provide a method and apparatus for avoiding the formation of an exhaust plume upon introduction of flue gases containing gaseous water to the atmosphere by raising the enthalpy of the flue gases above that at which the gaseous water will condense regardless of the particular ratio between flue gases and outside air.

It is still a further object of this invention to provide a method and apparatus for avoiding the formation of an exhaust plume upon introducing flue gases containing gaseous water vapor to the atmosphere wherein the method is particularly characterized by its economy and efliciency in use.

Further purposes and objects of this invention will appear as the specification proceeds.

We have now discovered a method and apparatus which surprisingly avoids the formation of an exhaust plume, such as is formed upon combustion of a hydrocarbon fuel in a gas space heater, when introducing flue gases containing gaseous water to the atmosphere by raising the enthalpy or heat content of the flue gases to a level above that at which the gaseous water condenses, regardless of the mixture ratio between the flue gases and the outside air prior to the time that the flue gases are introduced to the atmosphere.

In the accompanying drawings, our invention is graphically shown, wherein:

FIGURE 1 shows the dilution of flue products with air on a psychrometric chart;

FIGURE 2 shows efficiency curves ploted against ambient temperature and relative humidity, illustrating conditions at which condensation is expected to occur;

FIGURE 3 shows one embodiment of the apparatus of the present invention, having a gas furnace for heating a dwelling unit wherein exhaust plumes are avoided by the controlled addition of enthalpy to the flue gases;

FIGURE 4 shows another embodiment of the invention;

FIGURE 5 shows a circuit diagram illustrating one method for controlling the auxiliary burner in FIGURE 3;

FIGURE 6 shows another circuit diagram for controlling the auxiliary burner of FIGURE 3; and

FIGURE 7 shows still a further method for controlling the auxiliary burner of FIGURE 3.

Although it had been believed that excessive heat would have to be added to flue gases to avoid plume formation, we have surprisingly discovered that the most efficient, economical, and practical method to avoid the formation of a visible exhaust plume is by increasing the sensible heat or enthalpy of the flue gases before they are introduced to the atmosphere.

Two conditions which have a great effect upon plume formation are the atmospheric temperature and relative humidity. Another factor of importance is the efiiciency of the furnace, which is also a measure of the exhaust gas heat content. It is possible to determine the amount of heat energy necessary to raise the enthalpy of the flue gas to a level to prevent condensation by determining furnace efficiency, ambient temperature, and relative humidity. From the theoretical standpoint, there are two variables which can only be estimated and which can be actualy determined only experimentally. One such variable is the actual relative humidity at which condensation starts. Condensation is a complex process and is affected by dust and chemicals in the atmosphere which act to start condensation at less than relative humidity. Thus, more heat is required to eliminate a visible plume if condensation begins at humidities below the theoretical relative humidity at which the plume is expected to form. Another such variable is the size of the droplet or crystal of condensed water and the size of the plume. The ability to see a plume is affected by light conditions, the size and concentration of the droplets, and the depth of the field of particle. Very small droplets and thin layers of droplets are invisible. Plume thickness cannot be predicted because the mechanism of mixing is an unsteady state process. Thus, depending upon the size of the droplet and lighting conditions, condensation may occur and yet not be visible. The effect of this is to require less heat to heat to eliminate plumes. This requirement of less heat thus offsets the requirement of more heat if condensation occurs at lower relative humidities. It has been shown by experiment that these two variables essentially cancel each other out.

Another important factor in plume formation is the flue gas, which will mix adiabatically or at a constant pressure with the air at a specified temperature and relative humidity. The enthalpy or heat content of the flue gas-air mixture is the sum of their individual enthalpies. This determines the temperature of the mixture. The water content or absolute humidity of the mixture is determined by the individual contributions of the air and flue gas mixture. The relative humidity of the mixture is determined by using the vapor pressure of water or ice at the mixture temperature. Condensation occurs whenthe relative humidity of the mixture exceeds 100%. With these factors, the amount of heat or enthalpy necessary to evaporate the water and heat the mixture to a lower relative humidity may be readily determined.

The described process may be most clearly visualized by referring to FIGURE 1, which is essentially a psychrometric chart. The flue gas is represented by point P which shows the enthalpy, humidity ratio, and temperature of the flue gas. The outside air is represented by point A. Any mixture of the air and flue gas must lie along the straight line which joins point A and point P. Condensation of the gaseous water contained within the flue gas occurs wherever the mixture line between A and F crosses the saturation curve shown on the chart. In order to avoid condensation of any mixture of the air and the flue gas, it is merely necessary to increase the enthalpy of the flue gas before introduction to the air where mixing occurs and thereby raise the enthalpy and the temperature of the flue gas to the point S. The heat content aof the flue gas is raised from an enthalpy of 11 to an enthalpy of I1 The temperature of the flue gas is simultaneously increased from t to t The dashed line joining point A to point S represents all possible mixtures of air and flue gas. However, this line never crosses the saturation curve so that condensation of the water vapor, regardless of the particular mixture of air and flue gas, cannot occur.

As a variation of the described method, if the point S is found to be at a higher level than is practical to heat the flue gas to avoid plume formation, condensation is eliminated by operating the burner or furnace at an aeration which is higher than normal, normal excess aeration being about 150% of the amount of air theoretically required. This is equivalent to partial dilution of the flue gas with air and the flue gas containing the excess air would, at the point D, be along the mixture line between point A and point F. The amount of heat necessary to increase the enthalpy from the point D to the point D along the dashed line joining point A and point S is significantly less than required to heat from point F to point S. Thus, by excessive aeration of the fuel, less heat is required for increasing the enthalpy of the flue gas to a level above that at which condensation can occur.

Referring to FIGURE 2, constant theoretical efliciency lines for furnaces are plotted against the ambient air temperature and the ambient relative humidity. The theoretical efliciency, as previously suggested, is also a measure of the temperature or heat content of the flue gas at the point F shown in FIGURE 1. The theoretical efliciency is defined as one hundred minus the percent flue loss based on the higher heating value of natural gas and flue loss is directly related to the temperature and heat content or enthalpy of the flue gases. In this way, the theoretical efiiciency of the furnace indicates the locus of the point F on the psychrometric chart. Condensation of the gaseous vapor occurs at any air temperature and air relative humidity combination which lies below a particular furnace efliciency line. The higher the furnace theoretical efliciency, the greater is the number of occasions when a plume can form since the flue gases are at relatively higher temperatures and have a relatively higher heat content with furnaces of lower efliciency.

Therefore, by reference to the chart shown in FIGURE 1 and to the chart shown in FIGURE 2, the temperature or heat content to which flue gases must be raised to avoid plume formation is readily determined. The actual methods used to increase the enthalpy of the flue gases may vary. One method is to provide a separate heater, as a gas heater or an electric heater, to heat the flue gases after they have transferred heat to the heat exchanger. Another method is to increase the temperature of the incoming air and/or fuel before they undergo combustion in the combustion chamber. A third approach is to use an oversized burner in the combustion chamber or to have an auxiliary burner in the combustion chamber from which heat is diverted to the other flue gases after passing the heat exchanger. In practicing the method, controls responsive to the ambient temperature and relative humidity can be provided to operate the auxiliary heater which heats the flue gases, to operate the heaters for the incoming air and fuel, or to operate the auxiliary burner or the oversize burner. Alternatively, the heat could be applied at all times.

A number of experiments were performed in the city of Chicago over a wide range of conditions. Temperatures ranged from to 59 F. The relative humidity ranged from a dry 16% at 26 F. to a humid 93% at 36 F. Furnace efliciencies ranged from 70 to 85% The experimental program was compared to the theoretical predictions and this is set forth in Table 1 below. Exhaust plumes are present only when the furnace efliciency exceeds the predicted threshold efiiciency for a particular temperature and relative humidity. The difler- I TABLE I.VISIBLE PLUMES PRESENT Efficiency, Efliciency, Efficiency, percent percent percent observed theoretical difference 77..) m 7 0- 4 NO VISIBLE PLUMES Efliciency, Efliciency, Efiieiency, percent percent percent theoretical observed difference at no m-m As indicated above, the addition of enthalpy to the exhaust flue gases, which is equivalent to a decrease in the operating efiiciency of the furnace, can be accomplished in several ways. If a furnace or burner is to be perfectly modulated, in response to the atmospheric temperature and humidity conditions, so that only suflicient enthalpy is added to the flue gases to barely eliminate plumes, the increase in fuel consumption would be minimal. As an example, under Weather conditions which are the average of those encountered in the Chicago area for the last fifteen years, a perfectly modulated burner would require only about 3% more fuel to completely eliminate plumes. However, to modulate a burner perfectly in response to change in temperature and humidity conditions, an electronic digital or analog computer would probably be required. The relative costs for a computer and for fuel gas are such that the use of a computer cannot be economically justified. In the absence of perfect modulation of the burner, a decision will haveto be made as to when and how much enthalpy is to be added to the flue gases to avoid the formation of plumes.

Generally, the question of when to add enthalpy to the flue gases can be answered in two ways: first, by continuously adding enthalpy to the flue gases whenever the furnace is in operation; and secondly, by adding enthalpy only at certain preset conditions of atmospheric temperature and humidity. The first approach is essentially one of operating the furnace at a sufficiently low efficiency so that plumes will not form at any expected atmospheric temperature and humidity conditions. For example, referring to FIGURE 2, if the furnace is operated at about efficiency, it can be seen that plumes will not be formed at such extreme weather conditions as -10 F. and 100% relative humidity or -20 F. and 60% relative humidity. Since weather conditions more severe than these are soldom encountered, operating a furnace at 10% efficiency would substantially completely eliminate plumes without the use of any controls. However, such low operating efiiciency for the furnace is clearly uneconomical. Therefore, it is advantageous to employ some means whereby the addition of enthalpy to the flue gases can be made to take place at approximately the time when outside weather conditions indicate the likelihood of formation of plumes. FIGURES 3 and 4 show two embodiments wherein the addition of enthalpy to the. flue gases are so controlled. FIGURE 3 shows a gas furnace for a dwelling unit having an auxiliary burner to increase the enthalpy of the exhaust flue gases, the auxiliary burner being operated in response to impulses from temperature and humidity sensors. FIGURE 4 shows another furnace wherein the gas supplied to the main burners in the furnace is in part controlled by a solenoid valve which is controlled by impulses from humidity and temperature sensors located outside of the structure.

In referring to FIGURE 3, a gas furnace is generally shown at 10 which is mounted inside of a wall 11 for a dwelling unit. A gas supply conduit 12 supplies gas to the furnace through a main gas valve 13, which is controlled by a thermostat (not shown) within the dwelling unit. The gas is supplied to a distributor 14 which distributes the gas to a number of burners 15 where the gas is burned to produce heat. The hot flue gases, after passing through heat exchangers to heat the dwelling unit, are collected in a flue gases collector box 16 and passed through an auxiliary burner box 17 and opening 18 to be exhausted to the outside atmosphere. The gas for burning in the auxiliary burner is supplied through an auxiliary gas conduit 19 connected to the gas supply line 12 at a point beyond the main gas valve 13. The gas for the auxiliary burner passes through an auxiliary gas valve 20, which is controlled in a manner to be described hereinafter, and I passes into the auxiliary burner. On the outside wall 11, there may be mounted a number of temperature and humidity sensing means, such as humidity sensor 21 with a protective rain shield and a temperature sensor 22 with shield. More than one temperature or humidity sensing I means may be employed, as will be described in connection with FIGURE 7 below. The impulses from these sensing means are then utilized in controlling the auxiliary gas valve 20, which is a solenoid valve, in a manner to be described below.

In FIGURE 4, an oversized furnace 10 is shown for heating the same dwelling unit as in FIGURE 3 wherein the additional enthalpy for the flue gases is obtained by supplying additional fuel to .be burned in the burners 15 of the oversized furnace. In this figure, the gas is supplied through conduit 12 to a pressure regulator 23 where the gas pressure is reduced to about 3.5 inches of water. The gas having a reduced pressure of about 3.5 inches of water is then passed through a parallel arrangement where on one side of the arrangement a pressure regulator 24 further reduces the gas pressure to about 2.5 inches of water, while on the other side there is located the auxiliary gas valve 20. Valve is a solenoid valve and it is controlled by the impulses from humidity and temperature sensors, as in FIGURE 3 above. In this manner, the fuel gas at a pressure of about 2.5 inches of water is supplied to the furnace through pressure regulator 24 while fuel gas at a pressure of 3.5 inches of water may be supplied to the furnace through the solenoid valve 25.

FIGURES 5, 6 and 7 show various specific methods and apparatus wherein the solenoid valve 20- for controlling the admission of gas into auxiliary gas burner 17 of FIGURE 3 may be operated by the impulses from the humidity and temperature sensing means located outside of the wall 11. In FIGURE 5, a simple circuit for opening or closing the auxiliary gas valve 20 is shown which employs only the impulse from a temperature sensing means 22. A normally open switch 25 may be energized by the impulse from the temperature sensing means 22 to close at a specific low preset temperature. When switch 25 is closed, the circuit is completed and the solenoid valve 20 is then opened to permit the fuel gas to be supplied to the auxiliary burner 17.

The circuit shown in FIGURE 6 diifers from that shown in FIGURE 5 in the addition of switch 26 which is responsive to the impulse from a humidity sensing means 21 and a relay amplifier 27. In this circuit, the solenoid gas valve 20 will not open until both the switches 25 and 26 are closed. Switch 26 may be preset to close at a specific high relative humidity.

Referring to FIGURE 7, it is seen that solenoid valve 20 in this circuit can open only when switch 25 and either switch 26 or 29 are closed. Switch 25 is responsive to impulse from temperature sensor 22 and switch 29 is responsive to impulse from temperature sensor 28 while switch 26 is responsive to humidity sensor 21 and relay amplifier 27. In this manner, the solenoid valve 20 can be opened either through an extremely low outside ambient temperature alone as indicated by temperature sensor 28, or valve 20 may be opened by a combination of the temperature and humidity conditions, as sensed by temperature sensor 22 and humidity sensor 21. When the arrangement shown in FIGURE 7 is employed to control the addition of enthalpy to the flue gases to avoid the formation of plumes, switch 25 is generally preset to close at a relatively high temperature, such as 40 F., while switch 29 is preset to close at a lower temperature, such as 25 F. Switch 26 may then be present to close at a relative humidity where, at the temperature set for switch 29 and at the particular furnace efliciency, plumes may form. For example, referring to FIGURE 2, if switch 29 is preset to close at 25 F., and furnace efliciency is normally then switch 26 should be preset to close at about 80% relative humidity. This is so because at, say, 26 F. the switch 29 would be open and switch 25 would be closed, so that switch 26 should be closed at a. humidity before plume-forming conditions are present.

Referring again to FIGURE 2, it is seen that when the ambient temperature is about 40 F., and the furnace is operating at about 80% efficiency (as do many household furnaces), there will be no plumes at any humidity condition. However, as the temperature is dropped below 40 F., plumes may be formed on very humid days, say having a relative humidity of over Therefore, if a simple control means, such as the temperature sensing means shown in FIGURE 5, is to be used to control the solenoid valve 20 for supplying additional gas to either an auxiliary burner 17 or to an oversized furnace, the switch 25 must be set to close at a relatively high temperature, such as 40 F. Although this would eliminate plume formation almost completely, it would entail a relatively high rate of gas consumption. For example, in the past fifteen years in the Chicago area, there has been an average of about 2980 hours per year when the outside temperature is at 40 F. or below. This would mean that the solenoid valve in FIGURE 5 would be open and additional fuel consumed for about 124 days out of the year.

The arrangement shown in FIGURE 6 may be employed to reduce the gas consumption by using a humidity sensor. If for example, switch 26 in FIGURE 6 is set to be closed when the outside relative humidity is at 50% or above, while switch 25 is set to close when the outside ambient temperature is at 40 F. or below, then no plume would form when the outside relative humidity is 50% or more and even at lower outside relative humidity, no plume would form unless the temperature drops to below 20 F. Again, using the weather data in the Chicago area for the past 15 years, with this mode of operation, there would be a saving on the gas consumption amounting to about 6% (as compared to the arrangement in FIGURE while plumes would form over a possible period of about 60 hours per year.

The preferred arrangement is shown in FIGURE 7 with switch 25 set to close at 40 F., switch 29 to close at 25 F., and switch 26 to close at a relative humidity of 80% or above. In this arrangement, the solenoid valve 20 would be opened by either a temperature of 25 F. or below or a combination of a temperature of 40 F. and relative humidity of 80%. From FIGURE 2, it can be seen that under these conditions, no plume would form when the main furnace is operating at an efficiency of about 80% or lower. Again, referring to the weather data in the Chicago area for the last fifteen years, the control system shown in FIGURE 7, preset at the conditions indicated above, would operate to open the solenoid valve 20 for an average period of about 1680 hours per year or about 70 days per year. This represents a substantial savings in fuel over the devices shown in FIG- URES 5 or 6.

The method and apparatus for controlling the addition of the enthalpy to the flue gases discussed above, pertains to the question of when the enthalpy is to be added to the flue gases. The question of how much enthalpy to add is a separate and distinct one. Thus, it is possible to compute the exact amount of additional enthalpy required to avoid plume formation at any atmospheric temperautre and humidity conditions, but such computation to find the minimum amount of fuel necessary to avoid plumes would require the use of expensive and complicated devices such as a digital computer. Therefore, it is more economical to preset the solenoid valve 20 in the embodiment shown in FIGURES 3 and 4 so as to add suflicient amount of enthalpy to the flue gases to avoid plume formation at all but most severe weather conditions which is likely to occur in view of past weather data. Again, referring to the weather data in the Chicago area for the last 15 years, it has been found that with a furnace operating at about 80% efliciency, there would be an average about 611 hours per year during which plumesmight form; at a furnace efliciency of 40%, only about 3 hours per year when plumes might form; and at a furnace efficiency of about 20%, plumes would be substantially completely eliminated. If the preferred arrangement as shown in FIGURE 7 is employed, the setting of the solenoid valve 20 to drop the furnace efficiency to 40% when valve 20 is open would increase overall gas consumption by less than 25%, whereas setting solenoid valve 20 to drop the furnace efliciency to 20% would increase gas consumption by over 70%. Therefore, if three hours of plume formation per year can be tolerated, the setting of solenoid valve 20 to obtain a resultant furnace efliciency of about 40% would be advantageous. This means that when solenoid valve 20 is in an open position, the furnace is using about twice its normal fuel consumption.

The temperature sensing means shown in FIGURES 3 and 4 can be any conventional thermocouple known and employed by those skilled in the art for sensing atmospheric temperatures. The humidity sensing means is preferably an electric resistance hygrometer, which contains a wire coated with a hydroscopic film containing lithium chloride which becomes more conductive as its equilibrium moisture content increases. In such a humidity sensing means, the ambient humidity determines the conductivity of the coating which in turn governs the amount of current flow. An example of such a sensor is one made by the Minneapolis Honeywell Company, Sensor Q464A and Probe 22Pl31. A relay amplifier may be used to amplify the signal fromsuch a humidostat to operate the solenoid gas valve. An example of such relay amplifier is Minneapolis Honeywell No. R7088C.

The invention has been described in detail with reference to particular and preferred embodiments thereof, but it will be understood that variations and modifications can be made within the spirit and scope of the invention as described hereinabove.

What is claimed is:

1. A device for avoiding the formation of an exhaust plume upon introduction of flue gases containing moisture to the atmosphere comprising, in combination: means for raising the enthalpy of said flue gases prior to their introduction to the atmosphere, control means operatively associated with said enthalpy raising means to cause the same to raise the enthalpy of said flue gases, and an atmospheric condition responsive means connected with said control means for actuating said control means at predetermined atmospheric conditions.

2. A device according to claim 1 wherein said enthalpy raising means is an auxiliary gas burner and said control means is an electrically operable valve for admitting fuel gas into said burner.

3. A device according to claim 1 wherein said atmospheric condition responsive means is a temperature sensing means connected with said control means for sensing the atmospheric temperature and for actuating said control means at a predetermined low temperature to thereby operate said enthalpy raising means.

4. A device according to claim 3 wherein said temperature sensing means includes a thermocouple.

5. A device according to claim 3 wherein said temperature sensing means is preset to actuate said control means at a temperature about 40 F. or below.

6. A device according to claim 1 wherein said atmospheric condition responsive means is a temperature sensing means and a humidity sensing means serially connected with each other and with said control means so that said control means is actuated at a combination of predetermined low temperature and high relative humidity.

7. A device according to claim 6 wherein said temperature sensing means includes a thermocouple and said humidity sensing means includes an electric resistance bygrometer.

8. A device according to claim 6 wherein said temperature sensing means is preset at a temperature about 40 F. or below and said humidity sensing means is preset at a relative humidity of about 50% or above.

9. A device according to claim 1 wherein said atmospheric condition responsive means is made of a first temperature sensing means and a second temperature sensing means serially connected with each other and with said control means, and a humidity sensing means serially connected with said first temperature sensing means and said control means but parallelly connected with said second temperature sensing means, said first temperature sensing means being preset at a relatively high predetermined temperature and said second temperature sensing means being preset at a relatively low predetermined temperature, whereby said control means is actuated either at a predetermined low temperature or a combination of a relatively high predetermined temperature and a predetermined relative humidity condition.

10. A device according to claim 9 wherein said first temperature sensing means is preset at a temperature about 40 F. or below, said second temperature sensing means is preset at a temperature about 25 F. or below, and said humidity sensing means is preset at a relative humidity of about or above, whereby said control means is actuated by either a temperature about 25 F. or below or a combination of a temperature about 40 F. or below and a relative humidity about 80% or above.

11. A device according to claim 1 wherein said enthalpy raising means is preset so that its operation will result in an overall efiiciency of about 40% for the furnace from which said flue gases issued.

12. A method for avoiding the formation of an exhaust plume upon introduction of flue gases containing moisture to the atmosphere comprising: detecting atmospheric temperature and relative humidity conditions, and selectively raising the enthalpy of said flue gases at predetermined temperature and relative humidity conditions to thereby avoid the formation of exhaust plumes.

13. A method according to claim 12 wherein the enthalpy of said flue gases is raised to a point corresponding to an overall efliciency of the furnace, from which the flue gases issued, to about 40% 14. A method according to claim 12 wherein said predetermined temperature and humidity conditions correspond to either (1) a temperature about 25 F. or below, or (2) a combination of temperature about 40 F. or below and a relative humidity about 80% or above.

References Cited UNITED STATES PATENTS 2,517,446 8/ 1950 Ryder et a1 431202 3,248,178 4/ 1966 Hoskinson. 3,320,906 5/ 1967 'Domahidy 1101 FREDERICK L. MATTESON, JR., Primary Examiner 15 ROBERT A. DUA, Assistant Examiner 

